It is known that in a rotor for a disc brake of a vehicle that causes brake squeal, the sliding portion of a rotor, that is the disc portion, is in a steady-state vibration mode in which loops and nodes of vibration in the rotor shaft direction emerge in an alternating fashion in the peripheral direction of the rotor. Although the number of nodes varies depending on the dimensions and shape of the rotor, the number is usually between two and ten in the case of a rotor having typical dimensions and shape.
Japanese Unexamined Utility Model Publication No. JU-A-62-97329 proposes that in order to restrain the occurrence of brake squeal by exciting or producing a vibration mode having a number of modes which make it difficult to cause brake squeal, a number of radial grooves extending to the outer periphery from the inner periphery of one side face of a rotor are formed at equal angular intervals. Further, a number of radial grooves are also formed on other side face of the rotor symmetrically with the radial grooves on the one side face of the rotor.
However, with this proposal, the number of nodes of the vibration mode excited or produced by the grooves formed on the one side face of the rotor is equal to the number of nodes of the vibration mode excited or produced by the grooves formed on the other side face of the rotor, whereby the vibration occurring in the vibration mode produced by the grooves formed on the one side face is promoted by the grooves formed on the other side face. Accordingly, a steady-state vibration mode is produced and brake squeal is highly likely to occur.
Thus, a need exists for a rotor for a disc brake which is able to avoid producing a steady-state vibration mode amounting to the occurrence of brake squeal.